A polymer electrolyte fuel cell (PEFC) is a device for obtaining electric energy by causing a hydrogen gas provided as a fuel and oxygen provided as an oxidizer to react with each other. A unit cell formed as this fuel cell is constituted by a membrane electrode assembly (MEA) which is formed of a pair of porous electrodes (porous supporting layer+catalyst layer) are opposed to each other with a polymer electrolyte membrane interposed therebetween, and which is sandwiched between a pair of separators in each of which a channel for supplying a fuel or an oxidizer is formed. Unit cells formed in this way are stacked to be used as a stacked-cell battery. Various uses of such fuel cells as power sources for use on vehicles, fixed use and portable/mobile use at an operating temperature of about 80° C. are being expected. The electrode reaction is shown below.Anode: H2→2H++2e−Cathode: 2H++1/2O2+2e−→H2O  (Formula 1)
At the anode (fuel pole), a fuel such as hydrogen or alcohol is oxidized to produce hydrogen ions (protons). The produced protons move in the electrolyte membrane toward the cathode (oxygen pole or air pole) together with water, while electrons reach the cathode via an external circuit. At the cathode, water is produced by reduction reaction of electrons and oxygen. At this time, the protons produced at the anode move with water molecules through the electrolyte membrane and, therefore, the electrolyte membrane is maintained in a wet state. The separator is exposed to a strong acid solution atmosphere at a temperature from room temperature to 100° C. because it is in contact with the porous supporting layer (carbon paper or the like) constituting the MEA.
The separator has the current collection function and the functions of separately supplying a fuel or an oxidizer and discharging a reaction product as well as the function of acting as a mechanical reinforcement at the time of stacking. The separator further has the function of releasing or uniformizing heat generated by power generation reaction.
Separator materials are roughly divided into carbon materials and metallic materials. As carbon materials, a piece of graphite obtained by machining a graphite block, a carbon resin molded piece, an expanded graphite molded piece, etc., exist. With such materials, there are problems described below. A graphite block is high-priced and a large number of cutting steps are required for cutting it. A carbon resin molded piece can crack easily. An expanded graphite molded piece has high gas permeability.
On the other hand, a metallic separator has high electric conductivity, thermal conductivity, mechanical strength and hydrogen gas impermeability. Further, the development mainly of a metallic separator using mainly austenitic stainless steel as a promising material on which machining for forming a channel for a raw material fluid can be easily performed and which therefore enables reducing the manufacturing cost and the thickness is being pursued. However, there is a problem that the metallic separator is low in corrosion resistance. That is, the electrolyte membrane is superacidic and the anode side is put in an oxidizing atmosphere at about 100° C. and the cathode side in a reducing atmosphere, as described above. Also, in the vicinity of the metallic separator, a reacting material and a reaction product contact and an uneven temperature distribution in the areal direction occurs. Therefore a local cell can form easily in the metallic separator and there is an extremely high risk of the metallic separator being corroded. Also, an acid produced by degradation/decomposition of the electrolyte membrane, for example, during use of the metallic separator in continuous operation for a long time further increases the possibility of corrosion. This acid not only corrodes and damages the metallic separator but also reduces the electric conductivity of the electrolyte membrane by eluted metal ions. Further, there is a problem that eluted metal ions are precipitated to reduce the performance of a precious metal catalyst such as platinum. It is, therefore, difficult to put the metallic separator into actual use.
As a means for solving these problems, a method of forming an electroconductive polymer coating on the surface of a metallic separator or a method of forming a corrosion-resisting metal coating layer such as gold or platinum plating is used. For example, patent document 1 discloses a metallic separator having a metallic base member on which a continuous channel is formed by pressing, and which is coated with a coating layer having high adhesion. According to this document, separation of the coating layer does not occur easily, and corrosion of the metallic base member can be prevented.
Patent document 2 discloses a metallic separator having an intermediate metal layer in which a flow channel can be easily formed by stamping, a corrosion-resisting metal layer provided on the outer surface of the intermediate metal layer, and a coating layer of an electroconductive agent and a resin binder formed on the surface of the corrosion-resisting metal layer. According to this document, the corrosion resistance of the metallic base member can be maintained.
Patent document 3 discloses, as an invention in an earlier application made public after the basic application for the right of priority of the present invention, a separator structure in which an electroconductive channeled plate and a metal plate are combined. Also, in patent document 3, the provision of a coating layer for preventing corrosion or for limiting the growth of a passive film over the entire surface of the metal plate or at least on the portion to be brought into contact with a meandering slot is proposed to prevent corrosion of the separator and reduce the contact resistance.
Patent document 1: Japanese Patent Application Laid-Open No. 2000-243408
Patent document 2: Japanese Patent Application Laid-Open No. 2003-272659
Patent document 3: Japanese Patent Application Laid-Open No. 2005-294155